Machinery's Handbook, 31st Edition
Powder Manufacturing Processes 1543 The main advantages of the process are: low capital cost compared to traditional melt- ing, forging, etc.; fewer operational steps compared with conventional rolling; avail ability of low-cost row materials; precision control of composition and ability to use high-purity metals; applicability to composite type mixtures or immiscible components that currently cannot be produced by the conventional methods; and adaptability and versatility, including the ability to roll multilayer structure. Sheet metal for controlled expansion properties of products such as for alkaline batter- ies, fuel cell electrodes; and for coins can be made by this process. Powder Extrusion : Extrusion, in general, is a process that forces metal to flow through a shape-forming die. (see Extrusion of Metals on page 1519). The powder is placed in a vacuum-tight sheet metal container, then heated and extruded, generally through a cy- lindrical die. As powder particles pass through the extrusion die, they undergo intense deformation and pressure, which bonds them together and removes porosity between the particles. Because there is a limit to the amount of reduction in cross-sectional area that can be achieved by extrusion, often the billet extruded from one die is then further extruded through a die of smaller diameter to eliminate any trace of porosity. The full- density billets produced in this way can then be cut to length, reheated, and used in a closed forging die to produce complex-shaped parts. This method is particularly useful in production of aluminum and titanium parts for aircraft. Extrusion of powder also is used to produce billets of specialty alloys of copper and tool steels, which are then machined or formed into finished products. Powder Forging: The purpose of forging, whether performed hot or cold, is to im- prove part performance. Densification is an essential part of the powder forging pro - cess. The powder forging (PF) process is performed in three steps, with the first two similar to normal powder metallurgy processing. A form is pressed as a conventional PM compact, called a preform . The mass, density, and shape of the preform are con- trolled closely to ensure consistency in the characteristics of the final forged part. The preform is sintered with particular attention paid to the reducing of nonmetallic inclu- sions. The sintered preform can be forged to full density in two ways: direct forging the preforms from the sintering temperature, or forging sintered preforms after reheating to forging temperature. The amount of deformation involved is sufficient to give a final density approaching very closely that of the solid material, and consequently, the me- chanical properties are comparable with those of material forged from wrought bar. Advantages of powder metal forging include the following: improved strength and density; high level static and dynamic properties; material flexibility, from low- to high-alloy steel; minimum weight fluctuations and reduced burr waste; high cost- effectiveness; lower tooling cost; and good dimensional accordance of forged parts. The powder forging process is used mainly in making parts for the automotive indus- try. Such parts can have inside and outside spline forms, cam forms, and other forms that require extensive machining. In addition to the well-known connecting rod, other applications include bearing races, torque converter hubs, and differential gear sets. Additive Manufacturing with Metal Powder: Another approach to making parts from metal powders is by additive manufacturing (AM). Additive manufacturing processes use an energy beam, such as a laser, or material jetted from an inkjet print head to bond powder particles into the desired shape, as described in a CAD file, usually layer upon layer. AM processes for metal are derived from rapid prototyping with plastics, de- scribed in the section Additive Manufacturing Plastics on page 611. With this ap- proach, parts can be made into shapes that cannot be made by conventional processes. For example, AM parts may have internal channels that are not straight or round in cross section. Also, parts can be made with details that are too fine or nested to be reached by conventional machining tools. Another advantage is that multiple parts can be consoli- dated into one part. Also, most of the powdered material is included in the part, so there is minimal material waste. For a discussion of metal AM processes, see METAL CAST- ING, MOLDING, AND EXTRUSION on page 1480.
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